In general, electrophotographic processes, methods for developing formed electrostatic latent images include a dry development process such as a cascade process and a magnetic brush process and a liquid development process using a dispersion of toner particles in a nonaqueous solvent having a high electrical insulation property and a low dielectric constant. The latter method is most suitable for faithful reproduction of fine images or intermediate tone because it can use finer toner particles than the former.
In general, a liquid developer's properties with respect to fixing images is governed by a coating agent and a dispersant. Of the two factors, the properties of the coating agent are the most important. Examples of coating agent include alkyd resins, linseed oil-modified alkyd resins, rosinic resins, hydrogenated rosinic resins, terpene resins, rubbers such as styrene-butadiene rubber and cyclized rubber, styrene resins, vinyl toluene resins, polyester resins, and vinyl polyacetate resins. However, most of these resins are disadvantageous in that their fixing properties are poor.
Furthermore the coating agent must have another important property. Particularly, when attached to or bonded to a coloring agent to allow the formation of toner particles, the coating agent must be capable of easily making a dispersion of finely divided particles and also capable of making the thus-dispersed particles stably present in a carrier liquid.
In conventional coating agents, a coating agent which fully satisfies the desired image fixing, particle dispersing, and particle stabilizing properties at the same time has not been known.
As previously described, the liquid development process is excellent in reproduction of fine images and intermediate tone as compared to the dry development process. However, the liquid development is disadvantageous in that when a liquid developer is allowed to flow at a high speed, the image formed exhibits a tail-like run at its downstream side. This problem can be somewhat reduced by reducing the flow rate of the developer. However, this is not necessarily a preferable solution, because the reduction of the flow rate causes an increase in the process time and a decrease in development density. It is therefore desired that this problem be solved by improving the properties of the toner particles.